1. Field of the Invention
The present invention relates to an automatic direction finding system.
2. Description of the Prior Art
One of the most critical problems confronting the users of automatic direction finding systems is to inexpensively and simply "direction find" (DF) an originating source of transmission of a received radio frequency signal and accurately display the processed received signal information indicating the direction from which received signals arrived.
Prior art direction finding devices have evolved through the art since the early 1930's when direction finding techniques essentially amounted to manual turning of a Finch Sense Loop to determine the null of the received radio frequency signal indicating that the originating source of the radio frequency signal was located on either side of the plane of the loop. After two or more direction fixes had been taken on the received radio frequency signal at different physical locations, an operator could triangulate the originating source of transmission of the received radio frequency signal.
Last prior art direction finding devices comprised a sense antenna utilized with a loop antenna making it possible to determine the sense of direction of the originating source of transmission of the received radio frequency signal.
Prior art automatic direction finding systems evolved with such systems utilizing two or more antennas and displaying the processed received information in a variety of different ways such as on an indicating meter indicating a left or right direction of signal arrival, electromagnetic coils operating a directional compass, bearing displays, autocompasses, cathode ray tube displays, etc.
Some prior art automatic direction finding (ADF) systems required physically large and expensive antenna arrays switched manually or electronically which were connected to a signal processing circuit to subsequently display the relative direction of the originating source of the received radio frequency signal. Other prior art automatic direction finding systems utilized complex and expensive electromechanical or electronic circuitry to process the radio frequency signal received by the direction finding antenna array to yield directional information of the originating source of the received radio frequency signal.
The prior art automatic direction finding systems generally required complex modifications to existing receiver circuitry thereby not realistically permitting the receiver to be utilized solely as a receiver, but only as a receiver specially designated for use with the automatic direction finding system. Some prior art direction finding systems required receivers to have two separate distinct receiver channels to process the received radio frequency signals. Such dual channel receivers were undesirable and expensive in addition to being impracticable for most direction finding needs.
Representative prior art patents disclosing automatic direction finding systems illustrating the prior art devices as discussed in the previous paragraphs are further elaborated on in the following paragraphs.
Jarvis, U.S. Pat. No. 2,586,342, discloses a tricoordinate radio direction finder receiver utilizing a commutating switch for controlling the switching of the energies picked up by three loop antennas so as to impress the energies in sequence upon the input circuit of the radio receiver. The electronics includes sample and hold amplifier circuits to retain the signal levels received by each antenna. The circuit of Jarvis does not involve the multiplexing of any signals onto the received signal but rather merely achieves periodic changes in amplitude in the received carrier which are sampled and stored in synchrony with the commutation among the three loop antennas. Such an amplitude sensitive technique is totally inappropriate for conventional FM receivers which have significant limiting in the IF stages. The technique is also difficult in AM receivers having fast acting AGC circuits.
Weill, U.S. Pat. No. 3,303,504 discloses an ADF system having switching circuits for deriving directional information from the radio frequency signals of quadrature directional loops and an omnidirectional sensing antenna. A reversing switch is placed in the two leads of each of the pair of loops and the loops are driven in succession to cause the resulting cardioid to aim successively in each of four orthogonal directions. A resolver and direction indication with four equally, angularly disposed field producing elements are provided. A freely rotatable pointer is responsive to the resultant field of the four elements. A square wave generator with flip-flops is provided for successively operating the four switches to connect the resolver elements to the detector in synchronism with the successive operation of the loop reversing switches.
Yello, U.S. Pat. No. 3,435,455, discloses a direction-finder navigation receiver having an antenna modulator for combining alternately in-phase and out-of-phase, in response to an applied switching control signal, the RF signals from an omni-directional sense antenna with those from a directional loop antenna to produce overlapping cardiod antenna response patterns. A transformerless output switching demodulator compares the magnitudes of the in-phase and out-of-phase signals to determine the relative bearing of a received transmission. A pair of audio switching stages contained in the demodulator alternately translates oppositely-phased detector output signals from the receiver circuits in synchronism with the alternation of the antenna patterns to form a full-wave D C signal.
Grillot et al., U.S. Pat. No. 3,564,548, disclose a method and apparatus for crystal controlled automatic direction finding which is typical of the prior art automatic direction finders utilizing the conventional two antenna concept of the sense antenna and the loop antenna (which sense the electric and magnetic fields respectively). While the sense antenna signal is brought into the receiver and applied to a first tuned circuit through a rejection filter, the signal picked up by the loop antenna is transmitted to a goniometer and then into the first loop rf amplifier.
Guion et al., U.S. Pat. No. 3,824,596, disclose an automatic sector indicating direction finder system which incorporates a receiving antenna arrangement incorporating four separate antenna patterns, each having a field pattern which is a function of the azimuth angle, the apparatus forming antenna signals which are then transferred through a plurality of antenna-sequencing relays to a dual channel receiver. The two outputs of the dual channel receiver are applied to phase detectors and amplitude detectors which determine the relative amplitude ratios. These ratios and the phase measurements are stored in memory momentarily and are then strobed from memory through a hard wired decoding logic circuit to an external display of sixteen sectors, all equal in size to indicate the sector location of a target.
The previously discussed prior art automatic direction finding systems have failed to provide an inexpensive and simple automatic direction finding system for use with an ordinary off-the-shelf communications receiver.